1. Field of the Invention
The invention relates to an FM demodulator comprising a limiter in which a frequency-modulated FM signal to be demodulated is limited, and a marker pulse generator generating marker pulses in dependence upon the limited FM signal, the marker pulses being applied to a low-pass filter whose output signal represents the demodulated signal.
In such FM demodulators the marker pulse generator is used to mark the zero crossings in the limited FM signal. Either all zero crossings are characterized by a marker pulse or only those which occur at a positive or negative edge of the limited FM signal. These marker pulses should possibly always have the same width in order that they cause minimal noise. The marker pulses are subsequently applied to a low-pass filter at whose output a signal occurs which is quasi a measure of the distance between the different marker pulses, thus also for the modulation frequency of the FM signal.
2. Description of the Related Art
The prior-art marker pulse generators can essentially be classified in two groups.
In the first group of these marker pulse generators, a monostable multivibrator, generally referred to as monoflop, is used for generating the marker pulses. In this case the requirement that the marker pulses should possibly always have the same width is satisfied, but such monoflops cannot be integrated in an IC with sufficient accuracy, as in the IC processes, both resistors and capacitors, by means of which the time constants for such a monoflop are generated, may exhibit considerable fluctuations of their values. For example, resistance fluctuations of .+-.20% for resistors and capacitance fluctuations of .+-.10% for capacitors should be taken into account. Overall, a time constant fluctuation of approximately 30% results in the case of IC manufacture. Temperature effects are then not taken into account. For the output signal of the FM demodulator, this means that, dependent on the specimen and spread in manufacture, values occur which differ by .+-.3 dB, which is not acceptable for most applications. An FM demodulator whose marker pulse generator operates with a monoflop is known, for example, from the handbook "Taschenbuch der Hochfrequenztechnik, Springer Verlag, 1969, pp. 018 to 019".
In a second group of FM demodulators, the marker pulse generator is implemented in such a way that clock pulses of a fixed reference clock signal are counted. After the occurrence of an edge of the limited FM signal, a marker pulse is started at the next pulse of the reference clock signal, the duration of this marker pulse being ended after a predetermined number of clock pulses of the reference clock signal. The drawback of this solution is that the marker pulses are not set right at the start of the edge of the limited FM signal but only when after the occurrence of this edge the next pulse of the reference clock signal occurs. Thus, the marker pulses have different positions, or in other words, they fluctuate with respect to their phase position. Due to this fluctuating phase position of the individual marker pulses, additional noise in the FM demodulator is generated, which noise cannot even be eliminated by means of alignment. Such an FM demodulator is known from the publication "IEEE Journal of Solid-State Circuits", vol. SC-21, no. 6, December 1986, pp. 916 to 922.